In oil and gas wells, boreholes are drilled into the earth. Various types of pipe are lowered into the borehole. For example, casing provides a lining that is along the walls of the borehole. A drill string is a length of pipe used to drill the borehole. Coiled tubing is also used to drill. After drilling, tubing is located within the casing; oil and sometimes gas is produced to the surface through the tubing. In addition, wells are subjected to workover operations for maintenance.
Occasionally, it becomes necessary to cut the pipe at a location inside of the borehole. For example, if coiled tubing is being used to drill, the end of the tubing may become stuck and cannot be removed from the borehole. As another example, in a workover operation, downhole equipment may become stuck. Such a situation typically arises in boreholes having a cork screw profile. The tubing is cut near the stuck point, enabling most of the tubing to be withdrawn and salvaged for use in other wells.
In order to cut pipe inside of boreholes, I have developed a radial cutting torch, which is described in my U.S. Pat. No. 6,598,679. The radial cutting torch has proven to be successful.
However, there are situations where the radial cutting torch does not work well. Such situations arise where the pipe is blocked or closed below the radial cutting torch. For example, coiled tubing is typically run into a well with a check valve that prevents back flow of well fluids into the tubing. When the radial cutting torch is lowered into the tubing for a cutting operation, it is positioned some distance away from the check valve. The radial cutting torch uses hot combustion fluids directed radially out to cut the pipe. When ignited, the torch creates a pressure increase, or pressure wave, inside of the tubing. In an open pipe, the pressure wave propagates down the pipe to the bottom of the well. In a closed pipe, the pressure wave reflects off of the check valve or other closure back to the torch. The pressure wave jostles the torch, causing the torch to move from its position. This in turn spreads the hot combustion fluids over a larger area of the pipe, in effect distributing the cutting fluids over a larger area of the pipe due to tool movement to the point where the pipe is not cut.
One solution would be to locate the cutting torch a sufficient distance away from the closure to mitigate the pressure wave. In small diameter pipe, such as coiled tubing, this distance must be great, resulting in waste, as a long length of pipe must be left in the hole.
Thus, it is desired to cut pipe close to a blockage or closure.